Chromium(II)-isophthalate 2D MOF with Redox-Tailorable Gas Adsorption Selectivity

Redox-active metal–organic frameworks (MOFs) are very promising materials due to their potential capabilities for postsynthetic modification aimed at tailoring their application properties. However, the research field related to redox-active MOFs is still relatively underdeveloped, which limits their practical application. We investigated the self-assembly process of Cr(II) ions and isophthalate (m-bdc) linkers, which have been previously demonstrated to yield 0D metal–organic polyhedra. However, using the diffusion-controlled synthetic approach, we demonstrate the selective preparation of a 2D-layered Cr(II)-based MOF material [Cr(m-bdc)]·H2O (1·H2O). Remarkably, the controlled oxidation of the developed 2D MOF using nitric oxide or dry oxygen resulted in modified porous materials with excellent H2/N2 adsorption selectivities.


■ INTRODUCTION
One of the key application areas of metal−organic frameworks (MOFs) involves selective gas adsorption aimed at separation of gas mixtures.To this end, ultramicroporous (pore apertures <0.7 nm) MOFs appear as promising materials for gas storage and separation due to very specific internal pore geometries often allowing for distinctive interactions with various guest molecules. 1,2−9 Alternatively, flexible MOFs appear as promising materials for H 2 separation based on selective adsorption due to possible structural transformations triggered by the presence of guest molecules. 7−13 Another research direction aimed at the development of cutting-edge functional materials involves harnessing their chemical reactivity for rational functionalization.For example, redox-active MOFs appear very promising with regard to modern applications such as catalysis, energy storage, selective gas adsorption, or sensors, but this research area is still vastly underdeveloped. 14,15In particular, MOFs involving redox-active metal centers are considerably rare, but several examples involving, e.g., Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Ce-based systems have been published as promising materials for modern applications. 15−22 The limited scale of scientific progress in this area is well demonstrated by comparison of Cr(II) and Cu(II)-based systems, which form isostructural molecular paddlewheel-type carboxylate units, yet the latter is represented by thousands of coordination polymer structures published so far. 23Apart from the coordination polymers, Cr(II)-based molecular coordination cages (also known as metal−organic polyhedra, MOPs) have been gradually developed ever since the pioneering reports by Bloch and co-workers 24 and Zhou and coworkers 25 (Figure 1a).−28 For example, similarly to the Cr(II)-MOFs, the Cr(II)-MOPs show excellent O 2 /N 2 adsorption selectivity.Interestingly, Zhou et.al.demonstrated that the gas adsorption properties of Cr(II)-based MOPs changed after exposure to oxygen, 25 which suggests that harnessing their redox reactivity might be a way to tailor their application properties.
In some cases, MOFs and MOPs are constructed using the same starting materials, which raises the issue related to the synthesis selectivity.For example, depending on the synthetic conditions, copper isophthalates are known to form 3D MOFs, 29 and 2D-layered MOFs involving both hexagonal (kagome-type) 30,31 and square grid frameworks 32 as well as molecular Cu 24 (m-bdc) 24 (m-bdc = 1,3-benzenedicarboxylate) cages. 33Typically, polymeric 3D and 2D copper isophtalates are prepared by self-assembly at room temperature using various solvent mixtures and/or molecular templates, 34−36 but some examples of solvothermal synthesis have also been reported. 32In turn, the synthesis of molecular Cu 24 (m-bdc) 24 MOP most often requires increased temperatures (around 80 °C). 33,37,38−28 Notably, all of the previously reported procedures involving Cr(II) salts self-assembled using isophthalates or their derivatives resulted in the formation of cage-type species. 24,25s part of our continuing research on the design, synthesis, and functionalization of MOFs 20,39−43 and following our previous findings on the preparation of Cr(II)-MOFs, 20 herein, we demonstrate that the selectivity of self-assembly of Cr 2+ species with isophthalate linkers can be shifted toward formation of the 2D-layered MOF material [Cr(m-bdc)]•H 2 O (1•H 2 O) using the slow diffusion strategy (Figure 1b).Moreover, we investigated the postsynthetic redox reactivity of the developed 2D MOF, which resulted in significant changes in its gas adsorption properties.In particular, we discovered that oxidation of 1 using nitric oxide (NO) or dry oxygen yields porous materials with excellent H 2 /N 2 gas adsorption selectivity.

■ RESULTS AND DISCUSSION
The diffusion-controlled synthetic approach to the preparation of MOFs is a very promising strategy, which involves selfassembly at very low concentrations of substrates achieved by slow diffusion of metal ions and linker species.This type of controlled reaction environment provides enough time and energy for the molecules to assemble into high-quality MOF crystals, which are challenging to obtain via alternative methods, as discussed in our previous reports. 20,41In particular, a lowtemperature diffusion-controlled approach could be a beneficial strategy for controlling the selectivity of self-assembly processes due to limited thermal energy of the reactants.To this end, we decided to apply the diffusion-controlled synthetic approach to the reaction system involving Cr(II) ions and isophtalate linkers (m-bdc), which could lead to the formation of MOFs or MOPs, as inferred by comparison to Cu(II)-based analogues.
The reaction between CrSO  S1 and S2).The individual MOF layers involve Cr 2 (O 2 CR) 4 paddlewheeltype units with water molecules coordinated to each of the metal centers (Figure 2a).The Cr−Cr distance of 2.2960(18) Å in 1• H 2 O is close to that observed for other chromium(II) carboxylate paddlewheels with axial water ligands reported previously showing Cr−Cr distances in the range of 2.33−2.36Å. 20,44 The chromium paddlewheel units are interconnected by isophthalate linkers forming grid-type assemblies, which are AAtype stacked with a layer-to-layer distance of 6.702 Å, slightly shorter in comparison to that observed for the isostructural  copper-based MOFs showing the layer-to-layer distance in the range of 6.74−6.79Å. 32,45 Due to the orientation of the linker backbone, the 2D layers of Cr(m-bdc)]•H 2 O involve two types of openings, which extend into 1D channels (pore limiting diameters: 5.59 and 3.84 Å; maximum pore diameters: 6.74 and 5.09 Å, calculated using the Poreblazer v4.0 software 46 ) perpendicular to the coordination polymer layers (Figure 2b).The phase purity of the prepared material was confirmed by PXRD (Figure S2).Notably, the increased reaction temperature (50 °C) was necessary for the efficient preparation of 1•H 2 O since analogous reaction conducted at room temperature resulted in the formation of a mixture of products with overall lower crystallinity, as evidenced by the PXRD study (Figure S3).
The presence of 1D channels in the crystal structure of 1•H 2 O prompted us to conduct gas adsorption experiments in order to evaluate the application potential of the material.The initial activation tests showed that relatively high temperature (200 °C) and vacuum treatment were required to remove solvent molecules from the pores, which yielded activated material 1.The nitrogen adsorption experiment of 1, conducted at 77 K, revealed a type I isotherm typical for microporous materials with a maximum uptake of 67 cm 3 /g of STP and a surface area of 197 m 2 /g (Figure 3a, red curve).The hydrogen adsorption experiments conducted at 77 and 87 K (Figures 3b and S7) confirmed the microporosity of 1 revealing the maximum uptakes of 80.5 cm 3 /g and 68.6 m 3 /g, respectively, and the zerocoverage H 2 heat of adsorption of 7.4 kJ/mol (Figure S10), which is slightly higher in comparison to the value 6.2 kJ/mol reported for Cr 3 (BTC) 2 MOF. 20Additionally, CO 2 adsorption experiments were conducted at −78, 0, and 20 °C, which showed 63.8, 40.4,and 32.2 cm 3 /g maximum uptakes, respectively (Figures 3d, S5 and S6).
With regard to the literature data, 16,20,25 the high redox reactivity of the chromium centers in 1 was expected, which could provide an opportunity to perform redox-based postsynthetic modification and potentially tailor the properties of the developed material.To this end, we selected three sources of oxidative species, nitric oxide (NO), dry oxygen, and air, and used them to oxidize the developed Cr(II)-based MOF, yielding three modified materials, 1-NO, 1-O 2 and 1-air, which were further investigated using PXRD, spectral analyses, and gas adsorption studies.S16).The PXRD analysis of the oxidized samples showed that the crystallinity of the studied material dropped significantly (Figure S4), i.e., sample 1-NO was essentially amorphous and samples 1-O 2 and 1-air retained the original long-range arrangement, but the observed reflections were broader and less intense in comparison to the crude 1•H 2 O.This observation clearly indicates that oxidation of Cr(II)-MOF results in the introduction of major structural defects.Moreover, scanning electron microscopy (SEM) images of materials 1 and 1-NO revealed that in spite of amorphization, the grain size and shape before and after oxidation were very similar (regularly shaped grains ca.10−50 μm in size).However, significant structural damage (fissures and cracks) was introduced to the material Gas adsorption experiments of the oxidized materials demonstrated promising improvements in the H 2 /N 2 adsorption selectivity in comparison to that of 1.The maximum H 2 adsorption uptakes of samples 1-NO, 1-O 2 , and 1-air were 73.2, 71.7, and 24.5 cm 3 /g (Figure 3b, green, purple, and blue curves), which correspond to ca. 91, 89, and 30% of the original capacity of 1, respectively, but their ability to adsorb N 2 at 77 K was essentially not observed (Figure 3a, green, purple, and blue curves).While in the case of material 1-air, the observed decrease in the gas adsorption properties can be attributed to the sample decomposition, the properties of the samples 1-NO and 1-O 2 indicate that in spite of the amorphization, the resulting material acts as a molecular sieve for the H 2 /N 2 mixture.Further H 2 adsorption experiments conducted at 87 K revealed maximum uptakes of 68.6, 56.1, and 58.8 cm 3 /g for 1, 1-NO, and 1-O 2 , respectively (Figures S7 − S9) and zero coverage isosteric heats of adsorption (Qst) for H 2 of 7.4, 7.2, and 7.6 kJ mol, respectively (Figure S10).Moreover, cyclic experiments of H 2 adsorption by 1-NO revealed excellent repeatability of the observed uptakes (Figure S11).Notably, despite the significant industrial interest in H 2 /N 2 separation, 7 only a handful of MOFs capable of molecular sieving of H 2 /N 2 mixture have been reported till date, including 2D 47,48 and 3D 49−51 systems.In order to compare the performance of the 1-NO material to the previously known MOF-based H 2 /N 2 molecular sieves (materials showing significant H 2 adsorption and negligible N 2 adsorption), we have selected the maximum H 2 loading capacity at 77 K as the benchmark parameter.As demonstrated in Table S3, 1-NO and 1-O 2 exhibit properties well within the range of the best currently known MOF-based H 2 /N 2 molecular sieves.
Further experiments involving CO 2 adsorption revealed that the performance of 1-NO, 1-O 2 , and 1-air dropped with respect to 1 in experiments conducted at 0 and 20 °C.The maximum CO 2 uptakes of 1-NO, 1-O 2 , and 1-air were 32.8, 34.6, and 24.4 cm 3 /g at 0 °C and 24.0, 25.1, and 17.9 cm 3 /g at 20 °C, respectively, which are in the range of 56−86% of the reference values observed in 1 (Figures S5 and S6).However, CO 2 adsorption experiments conducted at −78 °C showed that both 1-NO and 1-O 2 overperformed the original material 1 exhibiting maximum CO 2 capacities of 72.7 and 74.6 cm 3 /g (Figure 3d).Finally, since the Cr(II)-based porous materials are known to have promising O 2 adsorption properties, 16,25 we measured the O 2 adsorption isotherms at 0 °C for 1, 1-NO, 1-O 2 , and 1-air materials.As a result, it was found that material 1 showed promising O 2 adsorption capacity of 25.7 cm 3 /g, but the adsorption process was not reversible, as evidenced by the repeated experiment reaching the capacity of only 3.7 cm 3 /g (Figure S12), which is similar to the results reported by Zhou for the Cr(II)-based MOP. 25 Moreover, the O 2 adsorption capacities of the preoxidized samples 1-NO, 1-O 2 , and 1-air showed maximum capacities in the range 3−4 cm 3 /g (Figure 3c).
Due to the low crystallinity of 1-NO, 1-O 2 , and 1-air materials, detailed structural investigations aimed at deeper understanding of the observed adsorption phenomena were significantly hampered.Nevertheless, in order to probe the structures of 1-NO, 1-O 2 , and 1-air, FTIR spectroscopy was employed.−55 These observations indicate that upon exposure to gaseous NO, the Cr (II) centers in 1 were oxidized forming the Cr (III) -NO species.Furthermore, the FTIR spectra of samples 1-O 2 and 1-air appear very similar, but their complexity hindered the unequivocal identification of the specific chemical structure of the chromium−oxygen species present in these samples.

■ CONCLUSIONS
In summary, we would like to emphasize that the redox-active Cr(II)-based MOFs show great potential for both basic and applied research, but their investigation poses significant experimental challenges, as reflected by only a handful of reports following the pioneering work by Long and coworkers. 16While previously reported attempts at self-assembly of Cr 2+ cations and isophthalate linkers led to the formation of molecular MOP cages, 25 our study shows that using the diffusion-controlled approach allows for directing the course of this process toward a novel 2D MOF product.Moreover, we developed a promising strategy for tailoring the gas adsorption properties of the Cr(II)-based MOF by capitalizing on its redox reactivity, which resulted in the preparation of a material with excellent H 2 /N 2 adsorption selectivity.Thus, the presented results provide valuable insights into the relatively unexplored field of Cr(II)-MOFs.They also demonstrate a promising strategy of MOFs' postsynthetic modification utilizing their redox reactivity, which could inspire more investigations in this promising research area.Further studies involving other redoxreactive MOFs are currently underway.

■ EXPERIMENTAL SECTION
Materials and Methods.All reactions and manipulations were carried out under an inert atmosphere of dry nitrogen using standard Schlenk and glovebox techniques.Water was prepared by repeated sonication, vacuum treatment, and N 2 bubbling in order to fully remove dissolved O 2 .THF was distilled off with sodium benzophenone immediately prior to use.Sodium hydroxide and 1,3-benzenedicarboxylic acid were purchased from Aldrich and used as received.Sodium 1,3benzenedicarboxylate was prepared by mixing appropriate amounts of NaOH and carboxylic acids in water.Chromium(II) sulfate was prepared according to a literature procedure. 56ynthesis of 1•H 2 O. Solutions of chromium(II) sulfate pentahydrate (690 mg, 2.9 mmol) in 2 mL of water and sodium 1,3benzenedicarboxylate [freshly prepared using 481 mg (2,9 mmol) of 1,3-benzenedicarbocylic acid and 232 mg (5.8 mmol) of NaOH] in 2 mL of water were prepared.The solutions were placed in a special glass reactor that prevented mixing of the substrates (Figure 1d).Next, the reactor was gently filled with water, creating a diffusion path between substrates.The diffusion-controlled process was conducted for 4 weeks at 50 °C, which resulted in the formation of red single crystals of 1•H 2 O.The product was collected, washed three times with water and THF, and dried in vacuo.PXRD analysis confirmed the phase purity of the product.Yield: 497 mg (73%); elemental analysis of X-ray Crystallography.The crystals were selected under Paratone-N oil, mounted on nylon loops, and positioned in cold stream on the diffractometer.The X-ray data for 1•H 2 O were collected at 100(2) K on a SuperNova Agilent diffractometer using CuKα radiation (λ = 1.54184Å).The data were processed with CrysAlisPro. 57Structures were solved by direct methods and refined using SHELXL-2016/4. 58All nonhydrogen atoms were refined with anisotropic displacement parameters.Hydrogen atoms were added to the structure model at geometrically idealized coordinates and refined as riding atoms.In 1• H 2 O, no satisfactory structural models for highly disordered solvent molecules could be assigned, and therefore, Solvent Masking in OLEX2 was used to remove the electron densities of these disordered species. 59XRD Analysis.PXRD data were collected using an Empyrean diffractometer (PANalytical) employed with Ni-filtered Cu Kα radiation (40 kV, 40 mA) using Bragg−Brentano geometry.In order to achieve air-free conditions for measurements of sensitive samples, Anton-Paar PEEK polymer domed sample holders were used, which were loaded in an Ar-filled glovebox.Samples were mounted on zerobackground silicon sample holders prior to measurements.
Gas Adsorption Analysis.Volumetric gas sorption studies were undertaken using a Micromeritics Instrument Corporation (Norcross, Georgia, USA) ASAP 2020 system.Approximately, 100 mg of the corresponding solid product was transferred to a preweighed sample tube and evacuated under vacuum at 200 °C on the gas adsorption apparatus until the outgas rate was <5 μmHg.All gases used were of 99.999% purity.Helium was used for the freespace determination after sorption analysis.Adsorption isotherms were measured at 77 K in a liquid nitrogen bath.Temperatures of 273 and 293 K for CO 2 isotherms were maintained with a thermostated external ethylene glycol bath.SEM Imaging.SEM imaging was performed on a FEI Nova NanoSEM 450 microscope with a field-emission gun utilizing ETD and TLD detectors.High-quality imaging was performed with electron beam energy varying from 1 to 2 kV.
Infrared Spectroscopy.Infrared spectra were collected using a Bruker Tensor apparatus equipped with an ATR accessory.Samples were prepared by mixing ca. 1 mg of MOF with nujol in order to prevent air exposure.Resulting suspensions were transferred to the ATR sample stage and scanned 16 times in the wavenumber range from 400 to 4500 cm −1 .
UV−Vis Spectroscopy.UV−vis diffuse reflectance spectroscopy was performed at room temperature using a UV-2600 Shimadzu spectrophotometer in the spectral range of 187−1400 nm.For the nonabsorbing reflecting material, BaSO 4 was used as a reference.The samples were prepared in an Ar-filled glovebox using a sealable polymer sample holder with a quartz window in order to prevent air exposure.
4 and Na 2 (m-bdc) in water conducted at 50 °C in diffusion-controlled conditions (using a custom-made glass reaction cell shown in Figure 2d) resulted in the formation of red crystalline product: [Cr(m-bdc)]•H 2 O (1• H 2 O).Single-crystal X-ray diffraction experiment revealed that 1•H 2 O crystallizes in the P-42 1 m space group and forms a layered 2D MOF structure (Figure 2 a−c, Tables

Figure 1 .
Figure 1.Overview of the products resulting from the self-assembly of Cr 2+ cations and isophthalate linkers: (a) MOP reported by Zhou and co-workers. 25(b) 2D MOF reported here.

Figure 2 .
Figure 2. (a) Paddlewheel [Cr 2 (OOCR) 4 ] secondary building unit in 1•H 2 O; (b) 2D MOF layer in 1•H 2 O showing two different-sized pores (in yellow); green area shows the paddlewheel unit; (c) AA-type stacking of the 2D MOF layers in the crystal lattice of 1•H 2 O; green area shows the individual 2D layer.Cr = blue, O = red, and C = gray, H atoms have been omitted for clarity; (d) custom-made glass reactor for the diffusion-controlled synthesis of Cr(II) MOFs.
Freshly prepared samples of 1•H 2 O were exposed to NO, O 2 , or air for 12 h at RT, which resulted in a gradual color change (Figure S1) from red to brown (sample 1-NO) or green (samples 1-O 2 and 1-air).The exposure of 1•H 2 O to NO followed by evacuation resulted in the sample weight increase by 11.2%, which is close to the theoretical value of 12.8% representing stochiometric 1:1 binding of the NO molecule to each Cr(II) center.The 1-NO sample composition was confirmed using elemental analysis, revealing the averaged formula as [1• 0.82 H 2 O • 0.91 NO].Furthermore, the exposure of 1•H 2 O to dry oxygen or atmospheric air followed by evacuation resulted in a sample weight increase of 2.5% and 5.4%, respectively, while the theoretical values calculated for [1• H 2 O•O 2 ] and [1•O 2 ] were 13.7% and 6.0%, respectively.Based on the elemental analysis, the averaged formulas of materials 1-O 2 and 1-air were calculated as [1• 0.45 H 2 O • 0.52 O 2 ] and [1• 0.92 H 2 O • 0.43 O 2 ], respectively.Materials 1•H 2 O, 1-NO, 1-O 2 , and 1-air were studied using solid-state diffuse reflectance UV− vis spectroscopy, which suggested that the color changes observed upon exposure of 1•H 2 O to NO, O 2 , or air were related to redox reactions of the chromium centers rather than formation of charge transfer complexes due to the absence of characteristic charge-transfer bands (Figure